1. Field of the Invention
This invention provides a modification to the rotor of a three phase, squirrel cage induction motor which allows the motor to be started at full line voltage with an appreciably lower current than is possible with a conventional rotor design.
This invention, in its preferred embodiment, is confined to the cage assembly and related parts of the rotor. No current or control voltage commutation to the rotor is required. No modifications are required to the electrical windings, the stator section, or the end bells.
The modification allows a NEMA class B or similar motor (that is, a normal torque, relatively low-starting current motor), to be direct line started (that is, started at full supply voltage) with a two hundred to three hundred percent or more reduction in starting current. A large horsepower, three phase motor could thus be direct line started at two hundred to three hundred and fifty percent of full load current rather than five to six hundred percent as is normally the case. After the internal change over to the running mode, there is no reduction in motor efficiency from that of a standard class B or similar motor.
This invention will, in many cases, alleviate the necessity of starting large motors with reduced voltage controllers.
2. Description of the Prior Art
A three phase squirrel cage induction motor has line-connected electrical windings in only the stator portion of the motor. A current is induced in the rotating rotor portion of the motor in much the same way that the primary windings in a transformer induce a current into the transformer secondary. In an alternating current, squirrel cage motor, the stator windings and stator steel act as the primary portion of the circuit, whereas the rotor steel and the cage act as the secondary portion of the circuit.
When the motor is at full operating speed, the revolving magnetic field produced by the stator cuts across the rotor conductor bars formed the cage, inducing a voltage in the cage conductor bars. The shorting rings on the ends of the rotor complete the path for the rotor current between the rotor conductor bars. This low induced voltage causes a high rotor current to flow. There is also a back emf (electromagnetic force) induced into the stator windings from the rotor which opposes the line current. The induced voltages in the rotor and the back emf in the stator windings produce an appropriate full load current on the incoming lines.
However, at start-up, a squirrel cage induction motor draws very high current because the induced voltages in the rotor are high and because there is negligible back emf opposing the stator current. Depending on load and motor design, the average starting current may be from four to six hundred percent of full load running current. This high current value during start-up is of considerable concern to both electrical installation engineers and electrical utility companies.
In present motor technology, attempts to reduce the starting current load fall into two broad catagories.
1. The cage portion of the rotor may be designed to reduce start-up line current. With a relatively simply design change of the cage bars which increases their resistance, the rotor reactance is altered, which reduces the starting line current. However, the result is a motor which has considerably less efficiency in full load operation. In most cases, the poor economics of the reduced efficiency offsets any advantages gained in reducing the starting current.
2. The most frequently used method of reducing high current on start-up of large induction motors is done with reduced voltage controllers. Reduced voltage controllers (or starters) will be one of five mechanisms used to reduce the voltage (or current) to the line taps of the motor. The five starter options include autotransformer starters, wye-delta starters, part winding starters, primary resistor starters, and solid-state starters. Though prices and installation complexity vary for each type of starter and application, each of the systems require high current starting gear which adds considerably to the cost of the installation and continued maintenance.
Whereas, in contrast to present motor technology, the modified squirrel cage of this invention achieves the reduction in starting current in a novel and economical way which eliminates the need for any reduced voltage starting equipment. The motor is direct line started at full voltage. However, the rotor has an internal regulating means which limits starting current during the rotor acceleration time period. After the rotor has reached approximate operation speed, the regulating means re-adjusts for normal operation.
This reduction in inrush current during starting is achieved by producing a rotor which has two or more resistance values for the rotor cage. At initial start-up, the resistance of the cage is at the highest value which limits the inrush current to acceptable levels. After the motor speed has stabilized, the rotor resistance is reduced to a level at which the motor runs efficiently for its most economical operation. Inasmuch as the rotor cage resistance can be altered at will, the cage may have higher design resistance resulting in a lower initial inrush current. At the same time, there need be no compromise in the normal operation efficiency of the motor.
A search of the body of information in the field indicates no prior claim for a segmented squirrel cage shorting ring (as will be defined later in the body of this description) for the purpose of achieving a variable cage resistance during motor running conditions. The segmented squirrel cage shorting ring relies on the high resistivity of the rotor steel to provide a high resistance squirrel cage for starting. After full motor speed is attained, the segmented squirrel cage shorting ring is shunted for low resistance running efficiency.
Within the prior art, various methodologies are employed to reduce the starting current of a squirrel cage induction motor. In the one case, an attempt is made to increase the resistance of the rotor bars by adding some resistive element, such as shown in Cox (U.S. Pat. No. 2,196,059) or Robinson (U.S. Pat. No. 3,335,308). In another methodology, the cross sectional shape and/or location of the rotor bar is altered to reduce starting current, such as illustrated by Schiff (U.S. Pat. No. 2,324,728), or Andresen (U.S. Pat. No. 3,496,397). In yet another methodology, some mechanical structure of the rotor is caused to physically change location in order to alter the reluctance of the rotor, such as shown by Johnson (U.S. Pat. No. 1,740,599) or Myers (U.S. Pat. No. 1,752,104). In a final methodology, an additional rotational element is introduced within the rotor such as described by Beaudry (U.S. Pat. No. 3,445,699).
None of the above prior art shows a segmented shorting ring, nor provides multiple and selectable, resistance values for the squirrel cage rotor. It is thus the intent of this description to show that the invention described herein is novel and provides advantages over all prior art in providing a reliable and uncomplicated means for reducing the starting current of a squirrel cage induction motor. This is accomplished by a segmented shorting ring in conjunction with a subsequent means of either mechanically or electronically causing the segmented sections to become electrically common.